The Rule
To be believable, characters should not all be the same.
Human beings—sentient beings, even—are not all the same.
Counter-Example
"Who Goes There?" by John W. Campbell, Jr (as Don A. Stuart)
"Who Goes There?" is meant here as a stand-in for the kind of Campbell-ian Science-Fiction where the lead characters' personalities are all "scientist" or "engineer", or occasionally "girl". This works, to a limited degree, in short stories or novellas, where the ideas (or action) is center stage. But it's no surprise that the movies are more successful, because actors bring character with them.
Commentary
Even in genre power-fantasies, the worst sort of pandering is "this extraordinary character is literally just you!" Or "literally just me!" which is the road to Mary Sue-dom. In this case, the world-conquering James Halliday is one of several author inserts, where the author has apparently taken no time to research how one of these captain-of-industry-type character actually believes.